Or, use an "L" network. Same number of components, and it will
even
match
impedances that call for negative component values in a parallel
tuned
circuit.

W4ZCB
======================================

Actually, with an exact 1/2-wave antenna, an L-network suffers from
the same disability as does the simple L and C tuned circuit.

This is why automatic antenna tuner manufacturers, which use
L-networks, state in their operating handbooks that exactly 1/2-wave
antennas should be avoided.

But it is not an important matter. Very few antennas are EXACTLY
1/2-wavelength in length. And it is a simple matter to shift the
resonant frequency up or down a few KHz.
----
Reg, G4FGQ.
============================================

It was incorrect of me to say that an L-network suffers from the same
disability as the parallel L and C tuned circuit. But it does suffer
from disabilities of a different sort.

As you say, Harold, the L-network can be tuned to either side of
antenna resonance without calling for impossible negative component
settings.

The input impedance of a 1/2-wave antenna at resonance is a pure
resistance of several thousand ohms. But only very slightly on either
side of resonance the series resistive component of Zin falls to a
much smaller value - and there arises a series reactive component
which can also rise to several thousand ohms. Things change very fast
versus frequency.

The automatic tuner manufacturers recommend avoidance of end-fed
1/2-wave antennas because very high values of switched inductance are
needed on the lower frequency HF bands to provide an impedance match
to 50 ohms. You will be aware, in automatic tuners, that both L and C
are switched.

It is uneconomic to provide large microhenry values of switched
inductance for use only on antennas which are rarely found, i.e. on
end-fed wires which happen to be exactly 1/2-wave resonant.

It is also uneconomic to provide a switched range of small value capac
itors, say from 0 to 20 pF.

Whereas, with an L & C parallel tuned circuit the coil is home-brewed,
it has high intrinsic Q, it is fixed in value, and tuning is done by
varying only the capacitor. Impedance matching to the transmitter is
done by tapping the transmission line up and down the coil turns.

It also transpires that the power efficiency of a parallel tuned L & C
matching circuit is higher than an L-network which does the same job.
That is because the circuit designer has control over choice of L and
C values. Whereas, with an L-match, the L and C values are
pre-determined by the values of the load and generator impedances over
which the circuit designer has no control.

There is an additional complication of L-network design. At certain
values of load and generator impedances, the location and values of
L's and C's in the circuit can suddenly change versus frequency. And
there is always a choice between 2 circuits which perform same job.

Anyone interested can download program L_TUNER from website below.

The antenna input impdances obtained from program TUNEHALF can be
inserted in program L_TUNER and the L and C component values compared
between one type of tuner and the other.

Remember that with the L-tuner both L and C are variable. With the
parallel L and C tuned circuit, in general, only C is variable and
impedance matching is accomplished by tapping up the coil. ( or by
varying the turns on a link-coupling which, by the way, confers other
advantages.)
----
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Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
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